1. Field of the Invention
The present invention relates to a conductive polyacetal composition and, more particularly, to a conductive polyacetal composition characterized by improved toughness and flexibility, as well as electrical conductivity, low viscosity and low moisture pick-up.
2. Description of the Related Art
Polyacetal resins, also known as polyoxymethylene (POM) resins, are engineering thermoplastics that have broad use, for example, as replacements for metals in a wide variety of applications. Polyacetal resins typically exhibit excellent mechanical properties, fatigue resistance, wear, abrasion and processability. In some of the resin applications, electrical conductivity is required.
Compounding polyacetal resin with a sufficient amount of an electrically conductive carbon black has been practiced as a method for imparting electrical conductivity to the resin. For example, electrically conductive polyacetal resin compositions are disclosed by Masarnoto et al., in U.S. Pat. No. 4,391,741; and by Kausga et al., in U.S. Pat. No. 4,555,357. One commercially available electrically conductive carbon black, which has been used to form electrically conductive polyacetal resins is KETJENBLACK.RTM. EC carbon black (a trademark of Akzo Nobel Chemicals, Inc., Chicago, Ill.). Typical conductive polyacetal compositions include an oxymethylene copolymer and about 6 percent, by weight, of the KETJENBLACK.RTM. EC electrically conductive carbon black. While these compositions provide excellent conductivity, the addition of the carbon black reduces the toughness and flexibility of the final molded product. In applications where higher flexibility is required, breakage can occur because of poor elongation and low practical impact strength. Moreover, the electrically conductive carbon black can increase the melt viscosity of the polyacetal resin and degrade the melt flow, which can make processing of the resin by injection molding techniques difficult. In addition, it is known that if the melt viscosity is too high, polyacetal degradation and associated formaldehyde emission during processing (such as compounding or injection molding) can result.
In the view of the above-noted problems associated with the use of electrically conductive carbon blacks in polyacetal resin compositions, lower quantities of generally superconductive types of carbon blacks such as, for example, KETJENBLACK.RTM. EC 600 JD carbon black (a trademark of Akzo Nobel) have been used. As disclosed by Kusumgar et al., in U.S. Pat. No. 4,828,755, which is incorporated herein by reference, the lower quantities of superconductive carbon black provide the composition with adequate electrical conductivity with less impact on the resulting processing and mechanical properties. A typical conductive polyacetal composition loading of KETJENBLACK.RTM.EC 600 JD carbon black is between about 3 and about 5 percent, by weight. It is noted, however, that the superconductive carbon blacks are generally characterized by a high structure level (high DBP absorption) and a small primary particle size (high surface area, BET N.sub.2). Dispersion and processing of the superconductive carbon blacks in the polyacetal composition is difficult, therefore, because of the small primary particle size, and the so obtained final polyacetal composition viscosity can be high due to the carbon black's high structure level and small primary particle size.
It is known to improve the impact strength and flexibility of polyacetal resins containing an electrically conductive carbon black by incorporating a polyurethane into the composition. For example, Kusumgar et al., in U.S. Pat .No. 4,828,755, disclose conductive oxymethylene polymer compositions having enhanced flexibility and toughness by the incorporation of elastomeric polyurethanes.